PBX is short for private branch exchange, a private telephone network used within an enterprise. Users of a PBX share a certain number of outside lines for making telephone calls external to the PBX. Most medium-sized and larger companies use a PBX as a less expensive alternative to connecting an external telephone line to every telephone in the company. In addition, it's easier to call someone within a PBX because the number to dial is typically just 3 or 4 digits.
PBX architecture is undergoing an important transition from circuit-switched to packet-switched transmission and coding. The future of voice communications, using PBX systems, seems to be headed toward use of the Internet Protocol (IP).
While the design architectures of today's IP telephone systems are diverse, the basic concept is that an IP telephone works like a PC. It has a LAN card with a MAC address and an IP address. It communicates with an IP PBX via the IP protocol. Therefore, the IP PBX no longer tracks phone numbers to physical devices. As a result, a phone can be easily relocated to any functioning port without re-punching the line or changing the PBX port setting. Essentially, you can move phones from port to port like a PC, because the IP PBX no longer looks for the phone number at a physical location, it searches for the phone's IP address across the network. Phone numbers are encapsulated into an IP packet, transported across the network using the TCP/IP protocol and then de-encapsulated at the PBX.
VoIP (voice over Internet Protocol) is a technology devised to utilize existing IP infrastructures to carry voice. This can be accomplished by using intelligent phones that have both a MAC address (NIC card) and an IP address. A speaker's voice is encapsulated into an IP packet and transmitted to the PBX. This allows the phone to be identified by an IP address instead of a phone number. The reason behind this is so that like a computer, a phone can be relocated easily to any location without the need for patching or PBX reprogramming. Essentially, since the PBX now communicates to the phone using IP, it will know what phone number to send and receive voice information regardless of location.